1. Field of Invention
This invention relates to methods and apparatus for calculation of T-wave alternans (TWA). Specifically, it relates to methods and apparatus for obtaining quantitative measurements of TWA from electrocardiogram (ECG) signals.
2. Prior Art
The introduction of digital electrocardiography and digital computing has enabled the clinical use of advanced signal processing techniques and the detection of subtle electrocardiogram features of clinical significance. Some of these features are not detectable by expert visual inspection but have proven to be important markers for serious heart illnesses [16]. An example of such a feature is T-wave alternans (TWA).
T-wave alternans (TWA) are beat-to-beat amplitude oscillations in the T-waves of electrocardiograms. Numerous clinical studies have demonstrated the link between these oscillations and ventricular arrhythmias.
Several methods have been developed in recent years to detect and quantify TWA, and use it as a non-invasive test to identify patients who are at increased cardiac risk. Timedomain analysis methods of TWA involve subtracting T-waves of even versus odd beats as in the commercially available MMA method [11].
Repolarization is the electrophysiological phenomenon associated with the recovery of cardiac cells after their excitation. T-waves in an ECG are the electrical manifestation of this repolarization process and may reflect electrical disturbances in normal electrophysiology associated with some cardiac diseases. Thus, T waves provide physicians with indicators of cardiac abnormalities and a means to assess therapy.
T-waves correspond to the ECG manifestation of the differences in action potential durations in the myocardium. The beginning of the T-wave is linked to the first cells that repolarize, and the end of the T-wave is defined by the last cells in repolarizing. The contour of the complete wave is directly related to the path of repolarization, and alterations have a counterpart in the shape of the T-wave. This phenomenon can occur on a beat to beat basis, as in TWA.
Ventricular repolarization heterogeneity has been demonstrated to constitute a risk indicator of possible malignant arrhythmias and sudden cardiac death [11]. In the past, measuring the duration of QT interval (or QT dispersion, QTd) was the only way to assess ventricular repolarization. However, the QT interval does not provide information about any abnormality of the repolarization sequence. Additionally, the QT interval is innaccurate due to measurement and QT interval normalization problems [6], and because of some technical drawbacks related to T-wave end detection [1].
In recent years, the research community has also developed methods to detect and quantify TWA. Some of the most widely employed methods in clinical practice are [8]:                Spectral methods. A time series is created by taking samples from consecutive T-waves, and then the Fourier spectrum is computed. Peaks at frequency 0.5 Hz indicate the presence of TWA [14].        Complex demodulation method. The same time series as in the previous case is demodulated and low pass filtered. Amplitude and phase of the alternans are derived from this filtered signal [12].        Correlation method. A single cross-correlation coefficient is computed for every ST-T complex against a representative for a heartbeat series. If the correlation index alternates for some consecutive beats, a TWA episode is detected [2].        Poincare mapping. Poincare maps are formed by plotting T-wave magnitude of alternate beats [16]. Semiperiodic signals such as TWA, appear as tight clusters. TWA magnitude is the intercluster distance.        MMA. TWA magnitude is obtained by means of the maximum absolute difference of even and odd heartbeat series averages computed at T waves or ST-T complexes [13].        